[DependencyInjection] Added '@@' escaping strategy for YamlFileLoader and YamlDumper

Added the possibility to to use ```@@``` as an escaping strategy for
parameters that should be treated as strings but start with ```@```
(i.e. safe mailer passwords).

I took the ```@@``` approach from the ```%%``` escaping strategy and thought
that would fit best, but I leave it open for discussion.

| Q | A |
| ------ | ------ |
|  Bug fix?  |  yes  |
|  New feature?  |  yes  |
|  BC breaks?  |  no  |
|  Deprecations?  |  no  |
|  Tests pass?  |  yes  |
|  Fixed tickets  |  #4889   |